Using variables from surrounding scope in macro expansion

[bch29]

I am trying to write a format! macro as a type implementing the Macro trait. I would like to be able to invoke it like this

let x = 1
let y = 2
format! "x'={1+x}, y={y}"

where the above would expand to

let x = 1
let y = 2
"x'=" ++ show (1 + x) ++ ", y=" ++ show y

I have written the code to produce this expansion, but when I try to use it I get errors telling me that (++), show, x and y are undefined. I have tried using let symbols = Symbols::new(); and

let mut compiler_db = env.vm.get_database();
let mut compiler = env.vm.module_compiler(&mut compiler_db);
let symbols = compiler.mut_symbols();

for symbol lookup (passed to parse_expr to parse subexpressions, and for getting the show and (++) identifiers).

I could make this work by having the user pass symbols in like this

let fmt = { show, (++) }
let x = 1
let y = 2
format! fmt "x'={x'}, y={y}" {x' = 1+x, y}

where the last line would expand to something like

let args = {x' = 1 + x, y}
in "x'=" ++ fmt.show args.'x ++ ", y=" ++ fmt.show args.y

but I believe this is less pleasant to use. Is it possible to achieve my desired syntax with a macro?

[Marwes]

I think if you create the symbols on the Symbols map passed in to expand after #896 it should work as you expect.

[bch29]

Thanks for the fast response to this! I have tried that branch and still can't quite get it to work. Passing that symbols map to parse_expr lets it pick up locals from subexpressions correctly, but I am still getting errors on show and (++). Am I doing something wrong by creating a show expression like this?

ast::Expr::Ident(ast::TypedIdent::new(symbols.from_str("show")))

[bch29]

I have opened a PR including my format! implementation at #897 since you have an open issue to add such a macro. This implementation exhibits the issue where show and (++) cannot be found.

[Marwes]

I don't think you should rely on show and ++ to be in scope for the macro. Instead expand it to be let { show } = import! std.show in let { (++) } = import! std.string in ...

[bch29]

I wasn't sure if it was possible to put macro invocations in the AST, so I have had it look up the import macro in the MacroEnv then expanded it. This works nicely.

[Marwes]

Ah, right. The expansion should work iteratively on the output of an expansion as well.